In the processing of video signals various kinds of filters are used in order to smooth out undesired signal components (disturbing signals) or to separate different components of a useful signal from each other. Low-pass, high-pass and band-pass filters are used, among others. Other filters, such as comb filters, for example, are especially suited for the spectrum of video signals arising from the line-by-line and picture-by-picture scanning or sampling that is available in television. The quantization of video signals arising from the time-wise (picture-by-picture) and vertical (line-by-line) sampling and also by horizontal sampling resulting from analog to digital conversion is observable in corresponding structures visible in the outputs of known digital filters for video signals.
The use of so-called median filters for digital video signals, already briefly referred to above, is also already known. Thus for example in British Patent 2 202 706 A there is described a video signal processing circuit for suppression of disturbing pulses which are produced by the scanning of dirty films. For this purpose corresponding video signal pixels of several successive pictures are supplied to a median selector. In addition to a corresponding signal of a picture from the middle of the sequence, there are also neighboring pixel values supplied from the middle picture to the median selector, in order to compensate for a change of picture content of the previous and of the following picture which is produced by motion. In the disclosure of that British patent, however, it is not undertaken to fit the filter constituted by the median selector to any peculiar properties of the video signals or to dominant structures of television pictures.
The use of median filters for video signals is further described in the book "Digitale Bildverarbeitung" by P. Haberecker, third revised addition, published by Hanser Verlag in 1989. Among other things there is mentioned there a two dimensional filter of which the structurerizing element--referred to hereinafter as a window--is cross-shaped. Both of the above-mentioned known filter circuits are proposed for suppression of transient disturbing pulses essentially affecting a single picture element (pixel). In these cases there still remains the problem of the difficulty of distinguishing, from each other, disturbing pulses and high frequency portions of the useful signal.